Japanese Patent Examined Publication No. 41143/1993 discloses that 4-[.alpha.-hydroxy-5-(1-imidazolyl)-2-methylbenzyl]-3,5-dimethylbenzoic acid and pharmaceutically acceptable salts thereof have pharmacological actions such as potent TXA2 biosynthesis inhibitory action, platelet aggregation inhibitory action and vasodilating action, and that they are useful for the prophylaxis and treatment of thrombosis, cerebral apoplexy, myocardial infarction, acute heart death, angina pectoris, hypertension, asthma, nephritis and the like. In addition, 4-[.alpha.-hydroxy-5-(1-imidazolyl)-2-methylbenzyl]-3,5-dimethylbenzoic acid, an optically active compound thereof and a pharmaceutically acceptable salt thereof have been found to be useful for the prophylaxis and treatment of diabetic complications, such as diabetic neuropathy, nephropathy, ophthalmopathy and arteriosclerosis (Japanese Patent Application No. 340161/1995).
This pharmaceutically useful 4-[.alpha.-hydroxy-5-(1-imidazolyl)-2-methylbenzyl]-3,5-dimethylbenzoic acid has an asymmetric carbon in the molecular structure. When a compound having an asymmetric carbon is developed as a pharmaceutical agent, the corresponding optically active compound is used in view of provision of a chemically pure compound, reinforcement of pharmacological activity, removal of side effect, lowering of toxicity, simplification of absorption, metabolism, distribution and excretion, improvement in solubility and the like. This in turn renders resolution of the optical isomer an important matter for the development of a compound having an asymmetric carbon as a pharmaceutical agent.
As a method for resolving the optical isomer of 4[.alpha.-hydroxy-5-(1-imidazolyl)-2-methylbenzyl]-3,5-dimethylbenzoic acid, Japanese Patent Unexamined Publication No. 215771/1990 discloses a method for resolving the optical isomer of 4-[.alpha.-hydroxy-5-(1-imidazolyl)-2-methylbenzyl]-3,5-dimethylbenzoic acid, which comprises ester-bonding hydroxyl group bonded to the asymmetric carbon of 4-[.alpha.-hydroxy-5-(1-imidazolyl)-2-methylbenzyl]-3,5-dimethylbenzoate and an optically pure chiral carboxylic acid compound, to give two kinds of diastereomers, subjecting the diastereomers to column chromatography and then to hydrolysis reaction.
However, this optical resolution by column chromatography requires a large amount of a filer, a large amount of a solvent and a large separation device for the small amount of a substance to be resolved. This method is effective for the separation of a trace amount, but when operability and cost are taken into consideration, is unsuitable for industrial scale mass production.
Therefore, there has been a demand for a practical and useful method for the resolution of the optical isomer of 4-[.alpha.-hydroxy-5-(1-imidazolyl)-2-methylbenzyl]-3,5-dimethylbenzoic acid.